Mechanical face seals are used on various types of machines and equipment, such as pumps, compressors, and gearboxes, for providing a seal between, for example, a rotating shaft and a stationary component such as a housing. Such mechanical seals typically include a pair of annular sealing rings concentrically disposed about the shaft and axially spaced from each other. Typically, one sealing ring remains stationary (e.g., engaged with the housing) while the other sealing ring rotates with the shaft. The sealing rings further include opposing sealing faces that are typically biased towards one another. Mechanical seals may be generally categorized as “contacting” or “non-contacting”. In contacting mechanical seals the biasing force is carried by mechanical contact between the annular sealing rings. In non-contacting mechanical seals a pressurized fluid film between the annular sealing rings carries the biasing force. Non-contacting mechanical seals may be subcategorized as “hydrodynamic pressure lubricated” or “hydrostatic pressure lubricated”.
In a hydrodynamic pressure lubricated mechanical face seal (also referred to herein as a hydrodynamic mechanical seal) the seal faces are provided with features such as grooves or vanes. Relative motion of the faces thus tends to draw the lubricating fluid into the interface between the seal faces and effectively pressurize the lubricating fluid film against the fluid being sealed (e.g., drilling fluid in downhole tools). The hydrodynamic lift (separation) of the faces is dependent on rotational speed, fluid viscosity, and the shape of the hydrodynamic features. Fluid viscosity is typically highly dependent on temperature. Such dependencies on speed and temperature tend to make it difficult to design hydrodynamic seals that meet the criteria required for typical downhole tools.
In hydrostatic pressure lubricated mechanical face seals (also referred to herein as hydrostatic mechanical seals) an essentially steady state fluid pressure is provided to the interface between the seal faces, for example, by remote pumps or energized accumulators. In a typical hydrostatic pressure lubricated seal, a radial taper is formed in the seal interface. The radial taper typically converges from the higher pressure fluid to the lower pressure fluid and acts to maintain a predetermined gap between the seal faces (the size of the gap being the primary deterrent to fluid leakage). Hydrostatic mechanical seals typically have a broader range of stable operation as compared with hydrodynamic mechanical seals. For example, hydrostatic mechanical seals are typically much less dependent on rotational speed than hydrodynamic mechanical seals.
In use hydrostatic mechanical seals typically require a stable pressure differential from the higher pressure sealed fluid to the lower pressure excluded fluid. Reversing pressure may be particularly harmful since it may reverse the direction of fluid flow. Such pressure changes may also change the radial taper such that it reverses convergence, thereby allowing contaminants into the sealing interface and compromising the sealing function. Accumulators, in particular, tend to be subject to sticking or fouling, which may cause loss (or reversing of) pressurization in hydrostatic mechanical seals. Such loss (or reversing) of pressurization often allows the excluded fluid to enter the seal interface and thus may result in premature failure of the seal assembly. In certain downhole tools, such as drill bit assemblies, drilling motors, rotational steering tools, measurement while drilling tools, turbines, alternators, and production pumps, such failure of the seal assembly often results in penetration of drilling fluid into the interior of the tool, which is known to have caused serious damage and/or failure of the tool.
Furthermore, remote pressurizing devices tend to be slow to respond to external pressure variations, for example, drilling fluid pressure spikes in a downhole drilling environment. Such pressure spikes have been observed to cause a pressure reversal in hydrostatic mechanical seals and therefore may also allow excluded fluid, such as drilling fluid, to penetrate into the interior of the tool.
Therefore, there exists a need for an improved hydrostatic mechanical seal assembly, in particular, an improved hydrostatic mechanical seal assembly including a pressure generating device that might provide improved robustness for use in downhole tools.